1. Field of the Invention
The present invention relates to a method of producing a grain-oriented electromagnetic steel sheet having excellent magnetic characteristics and, more particularly, to a low iron loss grain-oriented electromagnetic steel sheet suitable for a material of iron cores used in transformers and other electric devices.
2. Description of the Related Art
A grain-oriented electromagnetic steel sheet for iron cores employed in transformers and other electric devices must have good magnetic characteristics and, particularly, a low iron loss. Iron loss is substantially the sum of hysteresis loss and eddy current loss. According to the conventional art, hysteresis loss is significantly reduced by, for example, using an inhibitor to highly integrate the crystal orientation in the Goss direction, that is, the (110)&lt;001&gt; direction, and reducing impurity elements which give rise to the pinning factor of the domain wall shift during magnetization. Eddy current loss can be reduced by many methods, such as increasing the Si content so as to increase the electric resistance of the steel sheet, reducing the thickness of the steel sheet, coating the surface of the base metal of the steel sheet with a coat having a coefficient of expansion different from that of the base metal to provide a tension for the base metal, and/or reducing the grain size so as to reduce the domain width.
Other methods for further reducing the eddy current loss have recently been disclosed, in which a steel sheet is grooved. The methods of forming these grooves can be divided into two main groups: methods in which grooves are locally formed on a steel sheet after the finishing annealing, that is, the secondary recrystallization, so as to achieve the demagnetization effect that reduces the domain size; other methods in which such grooves are formed on a steel sheet before the finishing annealing.
The former group of methods employs various processes for forming such grooves. For example, a process is disclosed in Japanese Patent Publication No. 50-35679 in which grooves are mechanically formed. Another process is disclosed in Japanese Patent Laid-open No. 63-76819 in which an insulating coat and a primary coat of a steel sheet are locally removed by laser irradiation followed by electrolytic etching. Still another process is disclosed in Japanese Patent Publication No. 62-53579 in which grooves are impressed on a steel sheet by a gear-shape roll and then annealed for removing the stress. However, the mechanical process and the process using a gear-shape roll form large amounts of burrs adjacent to the grooves, thereby significantly degrading the space factor of a final product such as a transformer.
Further, because the process in which the coating of the steel sheet is partially removed by laser irradiation followed by electrolytic etching after the secondary recrystallization requires another step of coating the steel sheet after the grooves have been formed by electrolytic etching, the coating thickness is increased, thereby degrading the space factor, increasing production costs and reducing productivity.
One method of the latter group in which a steel sheet is grooved before finishing annealing is disclosed in Japanese Patent Laid-open No. 59-197520. This method is free of the above-stated drawbacks, but fails to achieve a reduction in iron loss that meets present needs.
To achieve a reduction in iron loss greater than those achieved by the above methods, Japanese Patent Laid-open Nos. 60-255926 and 61-117284 propose a method in which after a finish-annealed steel sheet is irradiated with a laser beam to locally remove the insulating coat and/or primary coat and then etched to form grooves, the grooves are filled with a substance different from the steel of the steel sheet.
However, this method also requires another step of coating the steel sheet after the grooves have been filled, thereby degrading the space factor of the product, increasing production costs and reducing productivity.
Japanese Patent Publication No. 54-23647 discloses a method in which some regions are processed so as to inhibit grain growth during secondary recrystallization. These regions are formed by processing a steel sheet after cold rolling or annealing for decarburization by a mechanical process, such as shot peening, a thermal process using an electron beam or the like, or a chemical process utilizing diffusion of, for example, S, Al, Se and Sb. This method enhances the magnetic flux density and reduces iron loss by directly controlling secondary crystallization. However, in industrial-scale production, the mechanical process, such as shot peening, will not easily introduce uniform stress into a steel sheet, and the thermal process using an electron beam or the like will require a large apparatus and, thus, increases production costs.
Although the mechanical process has advantages in that the compounds of S, Al, Se or Sb can be applied to a steel sheet at a low cost by, for example, high-speed printing, this process also has problems. For example, while a steel sheet is being conveyed at a high speed, the substance applied thereto may well be blown off, causing variations in the amount of the remaining substance. Further, the substance applied to a steel sheet is liable to rub off while the steel sheet is being coiled up. No matter which of the processes is employed, this method causes a large dispersion of the magnetic characteristics of the products.
Japanese Patent Publication No. 63-1372 discloses a method in which, prior to finishing annealing, a surface of a steel sheet is locally processed and a dilute aqueous solution is applied thereto so as to control the secondary recrystallization rate. The local surface processing is plastic processing by using a ridged roll or irradiation with an electron beam or a laser beam so as to introduce stress which promotes diffusion of the substance applied thereto. However, the stress thus introduced is non-uniform and, therefore, causes non-uniform diffusion of the substance, resulting in variations in the magnetic characteristics.